Nerves supplying the internal anal sphincter: an immunohistochemical study using donated elderly cadavers

Area of Science:

• Anatomy
• Neuroscience
• Urology

Background:

• The nerves serving the internal anal sphincter (NIAS) are thought to arise from the pelvic autonomic nerve plexus.
• Their precise anatomical course and fiber composition have not been clearly defined.

Purpose of the Study:

• To elucidate the topographical anatomy and fiber components of the NIAS.
• To identify the origin and pathway of the NIAS in relation to pelvic structures.

Main Methods:

• Histological analysis of ten elderly donated cadavers.
• Immunohistochemistry was used to identify S100 protein, neuronal nitric oxide synthase (nNOS), vasoactive intestinal polypeptide (VIP), and tyrosine hydroxylase (TH).

Main Results:

• NIAS originate from nerves posterolateral to the prostate (males) or in the lower paracolpium (females).
• These nerves travel inferiorly along the levator ani muscle, joining the myenteric plexus above the epithelial junction.
• NIAS comprise nNOS-positive (parasympathetic) and TH-positive (sympathetic) fibers, with fewer VIP-positive fibers.

Conclusions:

• NIAS may be damaged during radical prostatectomy, tension-free vaginal tape insertion, or low anterior resection due to their anatomical course.
• The NIAS have a higher sympathetic fiber proportion than the large intestine's myenteric plexus, but their function is unknown.
• Post-surgical evaluation of sphincteric function is challenging due to complex nerve control mechanisms independent of direct nerve supply.